A really good opening could be shoujo no gensou / 少女の幻想. Might be a bit too scary 0_0 depending on how the go with the series. This song without words scares me LOL. The illusionary world kinda scared me XD.
Maybe if no secondary routes it would be good.
http://www.youtube.com/watch?v=pQs4esnWC88 to have a hear ^-^ (wait it has vocals lol)